1. Field of the Invention
The field of the present invention is that of optical systems used in the transmission and/or amplification of optical signals.
2. Description of the Prior Art
Systems of the above kind conventionally include a few hundred or a few thousand kilometers of transmission optical fibers, depending on the application, divided into sections connected by repeaters in which the optical signals transmitted are amplified and sometimes reshaped. Optical signals are transmitted in fibers simultaneously on a plurality of optical channels. The signals are therefore not all subjected to the same optical treatment and stresses. The optical repeaters therefore include different optical modules for processing some or all of the transmitted signals. For example, a repeater can include amplifiers (fiber amplifiers or semiconductor amplifiers), chromatic dispersion compensators, gain equalizers, multiplexers and any other module that the person skilled in the art may deem to be required.
Transmission optical fibers generally have a monomode or multimode core for the simultaneous propagation of a plurality of optical signals surrounded by a cladding protected by a polymer coating. The core and/or the cladding can be of silica or of a polymer plastics material, depending on the application. The fibers have a signal attenuation and a pass-band suited to the applications in which they are used.
An optical repeater generally includes two stages of optical amplifiers, and often a gain equalizer, multiplexer, dispersion compensator or other optical module between the two stages. These modules introduce losses, which can generally reach 9 dB, and their parameters are generally set by a specification during design and installation of the transmission line.
Optical systems are often subject to change and it is not uncommon for the parameters of an optical module no longer to be suitable for current transmission spectra. For example, the various optical components constituting the modules are subject to aging, localized work may be carried out on the line, or optical transmission channels or modules may be added after the line is installed. The parameters of the modules previously set then become unsuitable.
Moreover, fiber amplifiers (erbium-doped fiber or Raman amplifiers) are associated with pump lasers whose performance is fixed once they are installed. Just like pump lasers, amplifiers are often standardized and are not necessarily well matched to the operating conditions under which they are used, and even more so to changes therein.
Moreover, optical amplifiers are often associated with gain equalizers for compensating amplification differences between the transmitted signal channels. The equalizers are designed to conform to operating conditions of the amplifier that depend among other things on the input power. The input power may vary along the transmission line, for example as a function of the length of fiber through which the signal has traveled. A variation of power at the input of an amplifier shifts its operating point and causes a mismatch of the associated equalizer filter. For this reason dynamic optical equalizers have been developed; they adapt to the operating conditions of the amplifiers at all points of the transmission line. Gain equalizers provide a dynamically adjustable attenuation as a function of wavelength.
It is therefore standard practice to provide adjustable modules in optical repeaters, in particular dynamic optical equalizer modules, to adjust the operating conditions of the optical modules along the transmission line. Similarly, using tunable pump lasers to modify the characteristics of the amplifiers of an optical system is also known in the art.
Adjustable modules of the above kind (and lasers) exist already and are well known to the person skilled in the art. The essential problem is that of controlling the adjustable modules to modify their parameters in order to select optimum operating conditions. Tunability may be provided at the level of the module or at the level of the optical component.
Existing control techniques necessitate the measurement of certain optical or performance parameters and applying appropriate control signals as a function of the measured parameters.
FIG. 1 relates to a first prior art control technique and shows diagrammatically a repeater 10 with two amplifier stages 3 and 4 and an optical module 5, for example a gain equalizer. An optical measurement is carried out, for example by means of an optical spectrum analyzer 7 such as an OPM (optical power monitor) or an OCM (optical channel monitor). The transmission spectrum can be measured optically before, after or between the amplifier stages 3 and 4. The measurement is fed back to a control unit, such as a local processor, which operates on the module 5 or directly on the optical component to adjust it as a function of fixed parameters, such as a gain template in the case of a gain equalizer module. This prior art technique necessitates measuring means 7 (for example a spectrum analyzer) for each repeater 10, which represents a non-negligible cost, and does not necessarily produce the optimum adjustment because it does not reflect all of the optical changes to the line, the template set for a given component not necessarily being the optimum at a given time. This control technique takes no account of the possibility of disturbances farther down the line.
FIG. 2 relates to another prior art control technique, and shows the same components identified by the same reference numbers. This prior art technique measures optical parameters at a given point of the line for action upstream thereof. For example, the adjustment of a given optical module 5 located in a given optical repeater 10 is controlled by a measurement effected by a spectrum analyzer 7 in a downstream repeater 10′ which is around ten repeaters farther along, for example. The adjustment control signal is then transmitted by supervisory channels CS in the transmission line which are reserved for control and command purposes and can be used for the above measurements and adjustments.
The above kind of technique reflects the actual transmission line constraints better, but is relatively greedy in terms of capacity. The supervisory channels are limited so as not to encroach on the wanted bandwidth and are essentially reserved for purposes other than optical module adjustment.
Moreover, the above control techniques are still based on the replication of optical parameters, such as a power spectral template or an optical signal to noise ratio (OSNR) template, which have a direct influence on the quality of the signal but which are defined on the basis of hypotheses that cannot generally be guaranteed throughout the life of a system, or even when it is installed.
The document EP0700178 discloses a method of adjusting a wavelength tunable source and filters in an optical system having a single transmission channel, the method including:                a step of measuring the quality of the optical signal at the output of the system as defined by an error function based on the eye diagram or the bit error rate (BER),        a step of sweeping the wavelength to adjust it for an expected reduction of the error function, and        a step of adjusting transmission characteristics of the filter as a function of the chosen wavelength.        
To be more precise, the evolution of the error function as a function of wavelength is completely characterized throughout the range of values thereof.
This method is not compatible with optimizing the performance of the system in operation, i.e. while the system is transmitting data, as it necessarily implies momentarily degraded performance.
The method disclosed in the above prior art document is not a reliable and powerful method of adjusting a dynamic module.
Moreover, the above method relates to the adjustment of parameters associated with only a single transmission channel, rather than relating more widely to selective adjustment of parameters of one channel as a function of other channels.
Moreover, all the control techniques previously described are unable to adjust a plurality of parameters of a plurality of remote optical modules as a function of each other to optimize the operation of the optical system as a whole, in particular if there is a large number of parameters and/or a large number of transmission channels and/or of modules.
Accordingly, the above technique cannot efficiently manage dynamic modules distributed along a transmission line. Transmission system performance could therefore be significantly improved by efficient dynamic control of optical modules distributed along a transmission line.
The object of the present invention is to propose a new technique for dynamically controlling one or more optical modules included in an optical system including a plurality of transmission channels on the basis of the optical signal at the output of the system for adjustment of optical parameters of one or more upstream adjustable modules, which adjustment is optimized in terms of efficiency (response time, output signal quality improvement, reliability, etc.).
In particular, the invention aims to adjust optical parameters of the modules distributed within a system, such as a transmission line, as a function of each other and on the basis of the optical signal at the output of the system.